This invention relates generally to the injection of pulverized coal into a blast-furnace. More particularly, this invention relates to a method and device for the injection of pulverized coal into a blast-furnace crucible encompassing the combined injection of pulverized coal and gaseous oxidant into a blast-furnace crucible by means of a lance emerging in a hot-air blast tuyere. This invention also includes a novel design for a lance and tuyere for implementing this combined injection method.
It is well known that the injection of pulverized coal into the hot-air blast, which is blown through the hot-air blast tuyeres into the upper portion of the crucible of a blast furnace, has many advantages. In particular, it increases the production capacity of the blast furnace and allows significant quantities of coke to be replaced by coal which is far less expensive.
The injection of pulverized coal is performed conventionally by means of an injection lance into the hot-air blast at a certain distance upstream form the nozzle end of the tuyere in the crucible. The pulverized coal is in suspension in an inert gas. The oxidant is either constituted by the hot-air blast, (which may or may not be enriched with oxygen) or by pure oxygen, brought in via a separate pipe close to the nozzle end of the lance. In the latter case, pure oxygen is used to form a primary fuel mixture with the pulverized coal at the outlet of the lance, and the hot-air blast constituting the secondary combustion air.
From the specification of German Patent DE-4,008,963 C1, a method is known for the combined injection of pulverized coal and oxygen into a blast-furnace crucible by means of a lance emerging with one end in a hot-air blast tuyere. The lance body comprises an inner tube for the pulverized coal and an outer tube forming, with the aforementioned inner tube, an annular conduit for the oxygen. According to the method disclosed in the abovementioned patent specification, the jet of pulverized coal is directly surrounded at its periphery by an annular jet of oxygen. Although this method is satisfactory at low flow rates, problems do occur when greater quantities of pulverized coal are introduced into the crucible.
One problem is that in order to work efficiently during the injection of pulverized coal into a blast furnace, it is necessary to produce as complete a combustion of the coal as possible in the turbulent zone in the immediate vicinity of the nozzle end of the tuyere in the crucible. When combustion does not take place sufficiently or rapidly enough before or in this zone, (which is the case when working with high flow rates of pulverized coal in a tuyere) large quantities of powdery residues from the combustion accumulate in the blast furnace and in the furnace filters. These residues considerably increase the resistance to the flow of the hot gases.
The difficulty in obtaining a complete combustion in the turbulent zone is due, on the one hand, to the short distance available and, on the other hand, to the high speed of the hot-air blast in the tuyere. During the extremely short time available for the combustion of carbon particles at the outlet of the lance, the following events must take place; (1) the compact jet of pulverized coal in suspension in a neutral gas must be broken up, (2) the isolated coal particles have to be reheated until the release of pyrolysis gases has taken place, (3) the pyrolysis gases have to be mixed with the fuel, (4) the ignition of this gaseous mixture has to take place and (5) the solid residues of the pyrolysis have to react with the oxidant in a heterogeneous oxidation reaction. One of the major problems of the injection of pulverized coal into the crucible is therefore to increase the kinetics of the execution of these combustion mechanisms described very briefly hereinabove.
It is obvious that there is a need for an improved and/or novel method for the combined injection of pulverized coal and a gaseous oxidant into a blast-furnace crucible, by means of a lance emerging in a hot-air blast tuyere, which would enable the yield of the combustion to be significantly improved. This is especially needed when working with high flow rates of pulverized coal which is desirable because of the much lower cost of the coal over coke.